Transformation Induced Plasticity (or “TRIP”) steels exhibit greater strengths and ductilities relative to other high-strength steels, both before and after plastic deformation. These features make TRIP steels ideally suited for use in fabricating automobile parts of complex shape, since such parts can be made stronger and lighter than if made with other high-strength steels. In addition, such parts exhibit superior crash performance because of their superior strength and ductility.
TRIP steels are primarily made up of three microconstituents: polygonal ferrite, bainite and a dispersed austenite phase supersaturated with carbon. Depending on the alloy content, martensite may also be present at room temperature. Plastic deformation of these steels, such as occurs during stamping or the like, causes the dispersed austenite to transform into martensite. Because this dispersed austenite/martensites phase is supersaturated with carbon, the steel is stronger than comparable high strength steels, both before and after plastic deformation. Moreover, because the transformed martensite is smaller than the retained austenite, there is a volume change that accompanies this transformation. This volume change increases the plasticity of the steel which in turn enhances formability, i.e., the ability to transform the steel into complex shape by plastic deformation. This volume change also enhances uniform ductility, i.e., the ability of the steel after plastic deformation to absorb additional applied stress through plastic flow such as would occur, for example, in a collision. The presence of high carbon martensite also provides the strength for these steels. The presence of retained austenite also improved crash worthiness by absorbing energy to transform into martensite.
The vast majority of sheet steel parts in automobiles and trucks are assembled by resistance spot welding (RSW), although laser welding is becoming increasingly popular. Both of these techniques are autogenous welding processes, i.e., processes in which no filler material is used. Unfortunately, when TRIP steels are welded, neither technique can produce welds exhibiting the same microstructure as the base metal being welded See, for example, Chen et al., Transmission Electron Microscopy and Nanoindentation Study of the Weld Zone Microstructure of Diode-Laser-Joined Automotive Transformation-Induced Plasticity Steel, Metallurgical and Materials Transactions A, Vol. 39 A, March 2008, pp 593-603. Although controlling the cooling rate of the weld has been suggested to overcome this problem (See, U.S. 2008/0203139), this approach is too cumbersome to be practical commercially. Moreover, autogenous welding is inherently limited in terms of deposition rates as well as the size of the gap that can be filled, since no filler material is used.